We are interested in genes expressed during development that regulate the choice of cell fate (eg. nerve, muscle, skin) by individual cells within the animal. Our model system is the nematode C. elegans (a small free-living worm) that is widely used for developmental studies because of its small size, ease of culture in the laboratory, simple anatomy, rapid proliferation, and genetic manipulability. The simplicity of this organism allows us to study gene expression in greater detail than is possible in more complex organisms, such as mice and humans. One class of genes we study encode proteins that function directly to bind DNA and regulate the expression of other genes. Specifically, we study proteins belonging to the Helix-loop-helix (HLH) family. HLH proteins have been shown in mammals to play critical roles in developing muscle and nerve cells. We have found that C. elegans has HLH proteins that are very similar to those studied in mammals. In fact, we have demonstrated that the C. elegans proteins can function to regulate muscle cell formation in mouse cells and that a mouse HLH protein can function to regulate muscle cell formation in C. elegans. Our goal is to understand exactly how these proteins function in this regulation. For example, what DNA sequences do they bind to, what other proteins do they interact with, and how do cells that are destined to become muscle cells turn on these critical regulatory proteins.